The present invention relates to wireless charging systems and, more particularly, to techniques for detecting when the receiver has been removed from a wireless charging system.
A conventional wireless charging system comprises a transmitter (TX) and a receiver (RX), where the transmitter wirelessly charges the receiver. In particular, the transmitter comprises TX circuitry that provides electrical power to a TX coil. The receiver comprises an RX coil that couples inductively with the TX coil when the RX coil is brought near the TX coil, such that electrical energy is transmitted from the TX coil to the RX coil to power and/or charge the RX circuitry, which typically includes a rechargeable battery.
In a conventional wireless charging system, the receiver periodically transmits (via the RX coil) short-range, wireless communication packets to the transmitter (via the TX coil), e.g., every 200-500 msec, that indicate the control error and output power. As long as the transmitter receives such communication packets from the receiver, the transmitter knows that the receiver is still present. If the RX coil is removed from the TX coil, then the transmitter will no longer detect the communication packets from the receiver. If the transmitter fails to receive a communication packet from the receiver for a specified number of continuous cycles (e.g., 5-6 cycles), then the transmitter determines that the receiver is no longer present, and the transmitter responds by powering down the TX coil.
After the receiver has been removed, but before the transmitter powers down the TX coil (e.g., 1-2 seconds later), if a different receiver is placed near the TX coil, then that newly placed receiver may be damaged by the existing high-power state of the TX coil. Furthermore, during that same interval, the radiating TX coil may cause EMC (electromagnetic compatibility) issues whether or not a different receiver is presented. Thus, it would be advantageous to have a transmitter that quickly detects removal of a receiver.